1. Field of Invention
The present invention relates to methods for improving the quality of a starch-containing food with an enzyme having a glycosyl transfer activity which transfers an α-1,4 bond to an α-1,6- bond. The present invention relates to methods for suppressing the retrogradation a starch-containing food with the enzyme. The present invention further relates to an agent for improving the quality of a starch-containing food, in which the agent contains the enzyme.
2. Discussion of the Background
When gelatinized starch is allowed to stand at ambient temperature or low temperature, it separates out water therefrom and becomes hard. This phenomenon is called retrogradation and many studies have been conducted for the retrogradation of starch. In order to suppress the retrogradation, it is usually necessary to maintain the temperature of the starch at 80° C. or higher, to dry quickly the starch so as to make the water content 15% or less, and to keep the starch alkalinized at pH 13 or higher, etc. There also has been known a method where saccharides (such as glucose, fructose and liquid sugar), soybean proteins, wheat gluten, fatty acid esters, polysaccharides (such as that from yam, konjak, etc.) thickeners, surfactants, enzymes, etc. are added to a starch-containing food, whereby the retrogradation is suppressed.
With regard to methods of utilizing enzymes, there has been disclosed a method of improvement of cooked rice where polished rice is mixed with enzymes such as amylase, protease and lipase, salt and cyclodextrin followed by boiling (see, Japanese Patent Laid-Open No. 58-86050); a method for the suppression of retrogradation of cooked rice where an aqueous solution of a saccharified type amylase (β-amylase, glucoamylase) is sprayed onto rice after cooking (see, Japanese Patent Laid-Open No. 60-199355); a composition for improvement of cooked rice containing cyclodextrin, one or more members of amylolytic enzymes, proteinases and cellulases, emulsifiers and water-soluble gelatin (see, Japanese Patent Laid-Open No. 59-2664); an additive for rice grains containing a combination of lecithin and an enzyme which decomposes albumen cell wall (cellulase) (see, Japanese Patent Laid-Open No. 02-72836); and an additive for rice grains using an endopeptidase (see, Japanese Patent Laid-Open No. 03-180151). The above-mentioned methods attempt to improve the quality of cooked rice by the addition of various enzyme preparations, but none of them achieves a remarkable effect.
With regard to cooked rice, it is the current status that, when rice of low quality (such as old rice, rice stored for two years and lowly palatable rice) is used, no satisfactory quality is achieved in terms of taste, texture, flavor, etc. even immediately after cooking. Further, even in the case of highly palatable rice and new rice, their commercial values (taste, texture, flavor, etc.) lower due to deterioration of the quality, such as retrogradation of starch with the passage of time, although the degree is different depending upon their storing conditions (storage in a rice cooker, room temperature, cold storage, freezing). Especially in the catering business of boxed lunches for taking out or the food service industry, rice cooked at a shop is sometimes kept warm for about 2 hours in a rice cooker, and there has been a demand for the suppression of of the deterioration of the quality of the cooked rice. Many methods have been investigated for improvement of the quality of rice upon cooking and for the suppression of deterioration upon storing.
For example, there has been disclosed a quality improving agent for cooked rice using trehalose (see, Japanese Patent Laid-Open Nos. 2003-12571 and 2003-225059). With regard to an improving method using enzymes, there has been disclosed an improving agent for quality of cooked rice using trehalose and amylase (see, Japanese Patent Laid-Open No. 10-234320); a method where rice is cooked together with protease, β-amylase and starch (see, Japanese Patent Laid-Open No. 07-274865); an improving agent for cooked rice where a pH adjusting agent and an adhesive derived from cereal are contained in amylolytic enzymes (α-amylase, β-amylase, glucoamylase), and an enzyme which decomposes the cell wall of rice grain (cellulase) (see, Japanese Patent Laid-Open No. 08-140600); an improving agent containing amylolytic enzymes (α-amylase, β-amylase, glucoamylase, etc.), proteinase (papain) and processed starch (see, Japanese Patent Laid-Open No. 07-031396). However, even in methods where various enzyme preparations are combined with saccharide or pH adjusting agent, it is the current status that a sufficient improving effect for the quality in terms of taste, texture, flavor, etc., such as a suppressive effect for the deterioration of the quality of a cooked rice product, which is kept warm for 2 hours in a rice cooker, has not been achieved.
With regard to bread, when time passes after baking, an inner part of the bread becomes hard and dry in texture. This change in the bread is referred to as retrogradation of bread. In a bakery product, such as a sandwich which has been markedly consumed in recent years, there are many cases where ingredients for refrigeration are used and, therefore, distribution by refrigeration and storage by refrigeration are becoming a mainstream. Usually, retrogradation of bread occurs under a refrigerated state, in which various methods have been investigated for suppressing the retrogradation.
For example, methods have been investigated to modify the formulations by adding an emulsifier or an enzyme, such as amylase, or by increasing the amounts of sugar, fat/oil or water in manufacturing bread. However, although there are methods where an emulsifier is solely added, there is also generally disclosed a method where an emulsifier is combined with an enzyme (see, Japanese Patent Laid-Open No. 05-168394); and a method where an emulsifier and protein are dispersed and compounded in fat/oil (see, Japanese Patent Laid-Open No. 08-26621), etc. However, although an effect for softness is achieved by those methods, a bad effect on texture caused by the emulsifier (such as where the product sticks to teeth, becomes glutinous or becomes a dumpling-like lump in the mouth) creates an unfavorable texture. In addition, although the prevention of retrogradation due to the enzyme preparations has been investigated in recent years, there is a weak effect for the prevention of retrogradation in the case of carbohydrase, such α-amylase, β-amylase and glucoamylase, whereby, when stored in a refrigerator, no sufficient effect is achieved. In the case of a proteinase, such as protease and papain, although an effect is available in terms of softness of the bread, the dough in the manufacture of bread becomes sticky causing problems that the process efficiency lowers or the sufficient volume of bread is not obtained.
In order to simplify the process of the manufacture of bread, frozen dough has been widely used in recent years. However, the thawed dough becomes soft due to growth of ice crystals, production of carbon dioxide, etc. during freezing storage and, further, the generation of gas lowers due to the death of yeast. Therefore, when bread is manufactured using such dough, problems such as a decrease in volume and a promotion of retrogradation occur.
As a method for improving the disadvantage caused by freezing and thawing of the dough, an emulsifier such as monoglyceride has been used. However, in the bread where the above-mentioned emulsifier is used, there are problems such as bad flavor due to the unique undesirable taste and smell, and that the texture is sticky. With regard to an improving agent using no emulsifier, there has been a proposal where an improving agent containing a maltotriose productive enzyme or hemicellulase is used for frozen dough (see, Japanese Patent Laid-Open No. 07-322811). In this method, however, there is a problem that the baked bread lacks in softness. There is also a proposal for a dough improving agent where hemicellulase and amylase are mixed with a fat/oil composition for increasing the dispersing property (see, Japanese Patent Laid-Open No. 2000-83573), but the effect is not so clear and, at present, it is not relate to any significant improvement in the quality of the dough.
With regard to noodles, it is noted that noodles are generally prepared by cooking raw noodles. Raw noodles are prepared in such a manner that an auxiliary material such as starch is added, if necessary, to powder such as wheat flour (strong flour, semi-strong flour, medium strength flour, soft flour and durum semolina flour), adlay flour, barley flour, rye flour, buckwheat flour, rice flour, corn flour or soybean flour, then mixed after addition of water together, if necessary, with salt, kansui(alkaline solution) or alcohol and spread, and the resulting noodle belt is cut into a predetermined shape. Noodles which are processed by freezing, frying, drying and pre-boiling the raw noodles prepared as such are also manufactured and distributed. Generally, the palatability of noodles is greatly dependent upon the physical property and texture. It has been demanded that those noodles have adequate hardness, elasticity, toughness and smoothness (slippery feel) after being cooked, have good texture with a good slippery smoothness through the throat and are hardly softened with hot water or by boiling. In order to manufacture the noodles as such, various attempts have been made, such as selecting suitable ingredients, the development of formulations, the improvement of manufacturing steps, the development of noodle manufacturing machines (see, Japanese Patent Laid-Open No. 05-316978), and an addition of food additives such as phosphates, emulsifiers or thickening polysaccharides or food materials such as egg white and gluten (see, Japanese Patent Laid-Open Nos. 54-76846 and 07-107934). However, the above-mentioned methods and additions are not sufficient, and there still has been a demand for a property improving agent by which the texture is able to be further improved.
With regard to an enzyme having a saccharide transferring activity which converts an α-1,4-bond to an α-1,6-bond, there may be listed transglucosidase and 1,4-α-glucan branching enzyme, etc., and there are disclosures for a method for the manufacture of seishu (sake or Japanese rice wine) using transglucosidase (see, Japanese Patent Laid-Open No. 54-157897); a method for the manufacture or mirin (sweetened sake) (see, Japanese Patent Laid-Open No. 01-171472); and a method for the manufacture of a cyclic glucan using a 1,4-α-glucan branching enzyme (see, Japanese Patent Laid-Open No. 08-134104). However, none of the above-mentioned mentioned methods relates to the suppression of retrogradation and an improvement of the quality of a starch-containing food, such as a cooked rice food and a processed wheat food.
In view of the foregoing, there remains a need for a method of improving the quality of a starch-containing food, and an agent for improving the quality of a starch-containing food, such that the degradation of the starch-containing food is suppressed and the quality of thereof can be maintained for long periods of time.